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Explain xkcd: It's 'cause you're dumb.
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Whenever I miss a shot with a sci-fi weapon, I say 'Apollo retroreflector' really fast, just in case.
Title text: Whenever I miss a shot with a sci-fi weapon, I say 'Apollo retroreflector' really fast, just in case.


In the comic, two people are engaging in a battle with laser guns. One appears to gain the upper hand as he jumps on a desk, as the other's shot goes wide. He delivers the classic line "Any last words?" and is answered with the confusing phrase "Apollo retroreflectors". The earlier wild shot, reflected off the moon, promptly lances down from space and hits him in the back.

A retroreflector is a device or surface that reflects light back at its source. The Apollo 11, 14 and 15 moon missions placed several such devices on the surface of the Moon to help scientists on Earth measure the distance between the two bodies using laser ranging. The likelyhood of the wild shot being aimed at the moon is fairly low in itself, and the probability of accidentally hitting a retroreflector on the moon is lower still.

The title text mentions that you would need to say "Apollo retroreflector" really fast, because from Earth you would only have about 2.5 seconds before the light is reflected back to its source.

It is worth noting that the number of this comic is 1441: a 'reflective' palindromic number.


[Two people engage in battle using handheld laser guns.]

[Person 2's shot misses, Person 1 jumps on a desk.]

Person 1: Any last words?

Person 2: "Apollo retroreflectors"

Person 1: What?

[Person 1 is hit in the back by the reflected shot.]

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